Ventilation is the physiologic process of moving a gas into (inspiration) and out of (expiration) the lungs of a patient, thereby delivering oxygen to organs of the patient and excreting carbon dioxide. During spontaneous ventilation, i.e. unassisted breathing, negative (sub-atmospheric) pressure is created within the chest of the patient. As a result, gas moves into the lungs of the patient.
In the practice of medicine, there is often a need to substitute mechanical ventilatory support for the spontaneous breathing of a patient. Mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator. Alternatively, the breathing of the patient may be assisted by a physician or other suitable person compressing a bag or set of bellows. In positive pressure ventilation, air (or another gas mix, e.g., oxygen mix) is pushed into the trachea of the patient. The positive pressure forces air to flow into the airway to expand and fill the lungs until the inspiration breath is terminated. Subsequently, the airway pressure drops, and the elastic recoil of the chest wall and lungs push the tidal volume, the breath, out through passive expiration or exhalation.
Mechanical ventilation may be necessary during respiratory failure or when patients are placed under anesthesia. Particular examples are patients with acute lung injury, including acute respiratory distress syndrome (ARDS); apnea with respiratory arrest, including cases from intoxication; chronic obstructive pulmonary disease (COPD); acute respiratory acidosis; respiratory distress; hypoxemia; hypotension including sepsis; shock; congestive heart failure; and neurological diseases such as Muscular Dystrophy and Amyotrophic Lateral Sclerosis; etc.